Polar Solvent Structure in the Debye-Hückel Theory of Strong Electrolytes

A general formula is derived for the potential energy of an ion in an ionic solution using all assumptions of the Debye-Hiickel theory except the assumption of constant permittivity. For the solvent the spatial correlation of polarization fluctuations is taken into account. The potential energy of an ion in it's ionic atmosphere is calculated for different models of the spatial correlation of polarization fluctuations and the corresponding excess free energy and activity coefficient of an ion is evaluated. It is concluded that the Debye-Hiickel theory using a constant value of e should only be valid for electrolyte concentrations lower than 1 0 7 mo l -dm 3 . The paper intends to show the consequences of dropping the assumption of constant permittivity and so encourage further efforts towards a more exact treatment of the problem.